As is known, the starter batteries used in motor vehicles for engine start-up and for powering electrical equipment are prone to gradual discharge and, with the engine running, the alternator compensates for their loss of energy. However, over time, battery efficiency is reduced, even in the presence of constant recharging from the alternator. This drop in efficiency is not usually detected, nor signalled to the user, and usually results in said users needing to address the problem when it is too late, i.e. when the battery has insufficient charge and/or functionality to start the engine. This situation causes great inconvenience, and is often made worse by the fact that users find themselves in places lacking a readily available battery replacement service.
It should also be considered that increasing numbers of vehicles are equipped with ‘Stop and Start’ technology. This includes devices which automatically switch off the engine during stops—in order to reduce atmospheric emissions of harmful gases, as well as to reduce fuel consumption. In these conditions a battery, in poor condition, may have difficulty in withstanding continuous engine restarts and the danger of a sudden enforced stop is therefore real. In an attempt to solve this problem a method and a device have been designed, by the same applicant, which enables the detection of battery efficiency, as described in Italian patent No. 1.357.179/2003. Such patent right, however, refers to the determination of the state of charge of batteries in open circuit conditions. The state of health of the battery, indicated by SOH (State of Health), is taken as an identification parameter of the battery condition and its value determines whether said battery is charged, uncharged or to be replaced. The SOH is a function of the state of charge SOC of the battery, directly determinable from the open circuit voltage of the battery itself and its degree of ageing, which results in quantitative terms, in an increase in the battery's internal resistance, which the method described in the above named patent measures through the application of a controlled current load under predefined conditions. This method, although valid, is more suitable for the realization of a bench device, given that it identifies the state of charge in open-circuit or key-off conditions.
Other methods and devices are also known for detecting the state of charge of batteries in key-on conditions, but these solutions are based on the measurement of currents, and hence result in complex and expensive equipment. Solutions of this type are known, for example, from U.S. Pats. No. 6,453,129 and 6,369,578 in addition to DE 19952693 and EP 0908737.